The present invention is a method for the preparation of hydrophobic fumed silicas which are useful as reinforcing fillers in rubber compositions. The method comprises two steps, where in the first step an aqueous suspension of fumed silica is contacted with an organosilicon compound in the aqueous phase in the presence of a catalytic amount of an acid to effect hydrophobing of the fumed silica thereby forming an aqueous suspension of a hydrophobic fumed silica. In a preferred method the first step further comprises the presence of a water-miscible organic solvent which facilitates the hydrophobing of the fumed silica with the organosilicon compound and the fumed silica has a BET surface area greater than 50 m.sup.2 /g. In the second step the aqueous suspension of the hydrophobic fumed silica is contacted with a water-immiscible organic solvent at a solvent to silica weight ratio greater than 0.1:1 to effect separation of the hydrophobic fumed silica from the aqueous phase. In a preferred process the hydrophobic fumed silica has a surface area within a range of about 100 m.sup.2 /g to 750 m.sup.2 /g.
Although hydrophobic fumed silicas prepared by the present method are useful in many applications such as reinforcing and extending filler in natural rubbers, thermal insulation, and as filler in floatation devices, they are particularly useful as reinforcing fillers in silicone rubber compositions. It is well known that silicone rubbers formed from the vulcanization of polydiorganosiloxane fluids or gums alone generally have low elongation and tensile strength properties. One means for improving the physical properties of such silicone rubbers involves the incorporation of a reinforcing silica filler such as fumed silica into the fluid or gum prior to curing. However, fumed silica has the tendency to interact with the polydiorganosiloxane fluid or gum causing a phenomenon typically referred to as "crepe hardening". A great deal of effort has been made in the past to treat the surface of reinforcing silica fillers with organosilanes or organosiloxanes to make the surface of the silica hydrophobic. This surface treatment reduces or diminishes the tendency of the compositions to crepe harden and improves the physical properties of the cured silicone rubber. Known processes for hydrophobing fumed silica contribute significantly to the cost of the fumed silica and therefore less costly methods of treating fumed silica are desired.
Lentz, U.S. Pat. No. 3,015,645, teaches the making of hydrophobic silica powders by reacting an organosilicon compound such as dimethyldichlorosilane or trimethylmethoxysilane with a silica organogel in the presence of an acidic catalyst to form a hydrophobic silica hydrogel. The hydrophobic silica hydrogel in the aqueous phase is contacted with a water-immiscible organic solvent to convert the hydrophobic silica hydrogel to a hydrophobic silica organogel which segregates into the organic phase.
Reinhardt et al., U.S. Pat. No. 4,072,796, describe a method where finely divided hydrophobic silica and silicates are prepared by precipitating of alkali silicate solutions with mineral acids or metal salt solutions and treated with organohalosilanes. The organohalosilane is a prepolycondensed organohalosilane or a mixture of prepolycondensed organohalosilanes.
Parmentier et al., U.S. Pat. No. 5,009,874, describe a method for making a hydrophobic precipitated silica useful as a reinforcing filler in silicone elastomers. In a first step the precipitated silica in aqueous suspension is hydrophobed with an organosilicon compound. In a second step a water-immiscible organic solvent is added to effect separation of the hydrophobic precipitated silica from the aqueous phase. The water immiscible organic solvent is added to the process at a volume(L) to weight (Kg) ratio of silica of from 1 to 5 and preferably from 1.5 to 4.5.
British Patent Specification No. 1,110,331 describes the hydrophobing of an aqueous suspension of fumed silica having a BET surface area of 50 m.sup.2 /g with a alkyl halogen silane and the subsequent flocculating in a water-immiscible organic liquid in quantities of from 0.01 to 1% based on the entire weight of the suspension. In the British Patent Specification the provided example describes adding 3 ml of toluene to an aqueous suspension comprising 300 g of hydrophobed silica which provides for a solvent to silica ratio of 0.01:1. The British Patent Specification does not describe the use of a polar organic solvent to facilitate hydrophobing of the fumed silica.